The lon mutants of E. coli have pleiotropic effects on cell growth: (a) they are defective in the formation of septa between dividing cells after treatment with ultraviolet light and therefore form long filaments, (b) they overproduce the mucopolysacharide coating of cells, possibly disrupting the control of operons involved in synthesis of mucopolysaccharide, such as the gal operon, (c) they are defective in the degradation of abnormal proteins. We are investigating the genetic defect in the lon mutants with the aim of determining the interrelationships of these defects. We have found that the product of an E. coli gene, su1A, which seems to negatively regulate cell division, is highly unstable in lon+ cells but is stable in lon- cells. This result suggests that the degradation of natural E. coli proteins may play an important role in regulating cell growth.